1. Field of the Invention
The present invention relates to foamed and expanded beads of a polypropylene resin for molding.
2. Description of the Background Art
Polypropylene resin foams are widely used as packaging materials, building materials, insulating materials and the like because they are excellent in properties such as mechanical strength and cushioning properties. As methods for producing polypropylene resin foams, there have been known methods such as a foam extrusion method and an expansion molding method. Of these, the foam extrusion method is a method suitable for continuously providing lengthy foams. However, only lengthy foams the shape in a section perpendicular to the extrusion direction of which are always the same, such as plate-like foams, columnar or prismatic foams, or those having projections along the extrusion direction on the surfaces of these foams, can be produced by this extrusion method. It is hence difficult to provide foams having a complicated shape. On the other hand, the expansion molding method is a method in which foamed and expanded beads (hereinafter referred to as "foamed beads" merely) of a polypropylene resin are filled into a mold and heated with steam to fusion-bond them to one another, thereby obtaining an expansion-molded article (hereinafter referred to as "molded foam") conforming to the mold. Therefore, a molded foam having an optional shape can be obtained according to the shape of the internal surface of a mold so far as it can be released from the mold. For this reason, the expansion molding method is widely used for the production of molded foams of various shapes.
In the expansion molding method, it is necessary for foamed beads filled in a mold to expand upon heating them so as to fill in voids among the beads (interbead voids), and moreover to surely fusion-bond to one another. Therefore, it is necessary to impart expandability to the foamed beads to be filled into the mold in order that the foamed beads can be expanded by heating. There is thus adopted a method in which the expandability is imparted to the foamed beads by compressing them into the volume smaller than that before their filling into the mold, and the compressed beads are then filled into the mold, or a method in which the expandability is imparted to the foamed beads by subjecting them to a pressurizing treatment with air or the like prior to their filling into the mold to increase the internal pressure thereof.
Of these methods, the expansion molding method making use of the foamed beads the internal pressure of which has been increased is preferred as a method for providing molded foams having a higher expansion ratio compared with the expansion molding method making use of the compressed foamed beads.
In recent years, there has however been a demand for provision of foams higher in mechanical properties such as compressive strength with the spread of polypropylene resin foams. In order to meet such a demand, a polypropylene resin having a higher melting point has come to be used as a base resin of foamed beads. When foamed beads of an uncrosslinked polypropylene resin, which are preferred in that they can be recycled, are used to conduct molding, however, it is impossible to provide a superb molded foam excellent in the fusion-bonded state of the foamed beads and little in interbead voids unless the pressure (temperature) of super heated steam (hereinafter referred to as "steam" merely) for molding as a heating medium is more raised as the melting point of the base resin is higher if the kind of the resin and the expansion ratio of the foamed beads are the same. In the foamed beads of the uncrosslinked polypropylene resin having a higher melting point, it is hence necessary to mold them using higher-pressure steam. As a result, such foamed beads have involved a problem that energy cost is increased, and moreover the molding cycle is lengthened, resulting in lowering of productivity.